Some of the most beautiful wood grain and patterning is found in what can best be described as a wart on the tree. There's no getting around the fact that burls (or burrs, as they are called in other parts of the world) look kind of nasty.
I haven't seen a convincing explanation of exactly why burls form, but some suggest it's a tree's response to beetle damage, others say it's triggered by a fungus. Setting aside what the cause may be, the product is a bump on the tree trunk which may grow to be far larger than the diameter of the tree hosting it.
Externally, it just looks like a big ugly bump.
But open it up and look inside ... it's not guaranteed, but often there is a explosion of wild grain, with swirls and eyes and jaw-dropping beauty.